The present invention relates generally to the field of mechanical systems, such as rotationally driven equipment. More particularly, the invention relates to a technique for more smoothly or accurately controlling movement of such mechanical systems to compensate for discontinuities in inertial loading.
A wide range of mechanical systems exist and are presently in use for driving various types of loads. Many systems include rotary machinery, such as engines or motors which drive loads either linearly or in rotation through the intermediary of various power transmission components. Such components may serve to appropriately locate a load with respect to a source (e.g. transfer position of application of forces) or may serve to alter the speed of the input source and, therewith, the reflected inertia of the load. In either case, it is generally desirable to provide for smooth control over a wide range of speeds and torques, with the ability to reverse directions of movement and hold accurate position where appropriate. In conventional power transmission systems, components interposed between a prime mover and a load may include various types of gear boxes, chain drives, servo drives, clutches, and so forth.
Depending upon the design and operation of the power transmission components in such systems, conditions may exist or develop in which the reflected load inertia is discontinuous. In a typical example, a gearbox, chain drive, or similar power transmission component, may exhibit a characteristic generally referred to as xe2x80x9cbacklash.xe2x80x9d Backlash may be considered a discontinuity in inertial loading due to a reduction in contact between two or more elements over a portion of a movement range. In a gearbox, for example, backlash may develop due to a loss of contact between neighboring gear teeth which generally intermesh to transmit rotary motion. The backlash may not be particularly noticeable when the load is driven. While the load is accelerating and decelerating the gear teeth are normally engaged. In most cases load friction keeps the gear teeth engaged even when the load is driven at a constant speed. However, when the system is held static, or when there is minimal friction applied to the load moving at constant speed, backlash may be result in sudden changes in the reflected load inertia which can lead to servo control instability. Another problem with backlash is the difficulty in maintaining accurate control in servo applications.
At present, servo tuning and adaptive control are key control technologies enabling drive and motion products to be easily applied to industrial motion control applications, particularly to power transmission applications. The objective of servo tuning algorithms, sometimes refer to in the industry as xe2x80x9cauto-tuningxe2x80x9d when applied to the motion control, is automatically to achieve a well tuned, xe2x80x9capplication-readyxe2x80x9d position or velocity servo loop. The servo loop generally refers to the control loop which is established to maintain a desired position or velocity as indicated by an input or command signal. Servo tuning is typically accomplished through foreknowledge of the motor-mechanical system or through the application of a minimal motion profile to an unknown motor-mechanical system. In either case, servo tuning algorithms result in servo loops that are generally tuned for a fixed load inertia.
One of the main objectives of adaptive control algorithms, sometimes referred to as xe2x80x9cadaptive tuning,xe2x80x9d is to maintain acceptable servo control performance levels despite significant variations in parameters associated with the motor-mechanical system. One common adaptive control problem that motion control engineers face is compensating for a variable system inertia, such as that present in discontinuous inertia situations such as that described above, and in particular in situations where backlash is present. Adaptive control algorithms compensate for these variations by continuously measuring the inertia of the system or by calculating the inertia based on the current position, time, or control state.
Traditional servo loop topologies do not make the task of developing auto-tuning or adaptive control algorithms a straightforward process. Servo control loops generally need to interface to a wide variety of drive power structures, motors, and mechanical systems, as well as to feedback devices. With traditional servo loop technologies, a variation in any of these components affects multiple gain parameters. When these variations occur in real time, such as due to discontinuities in inertial loading, modification of multiple gain parameters using adaptive control is awkward.
There is a need, therefore, for improved approaches to control of systems in which varying or inconsistent inertia loading may be present. There is a need, at present, for a straightforward approach to the control of loads in mechanical systems in which there is variation in load inertia. In particular, there is a need to compensate for discontinuous variation in load inertia as a function of position such as exhibited by mechanical backlash.
The present invention provides a novel control technique designed to respond to these needs. The technique is particularly well suited to the control of rotational systems, such as motor-driven loads including gear boxes, chain drives, ball-screws, or any other power transmission components which may be subject to discontinuities in load inertia. However, it should be borne in mind that the invention is susceptible to application in a more wide range of settings. That is, linear or other power transmission applications may also benefit from the technique, particularly where inertia loading may vary or where servo systems may require rapid adaptability to changing loads or speeds. In an exemplary embodiment, the invention is applied to a power transmission system including a gear box that exhibits backlash. Other systems, however, may exhibit similar control issues, and the invention is intended to address similar problems in all such applications.
In accordance with one aspect of the invention, a method for controlling a mechanical system exhibiting backlash includes generating control signals for application to an actuator in accordance with a first gain level. The first gain level is applied when a normal inertia load is applied to the actuator. The gain level is then reduced to a second level when a reduced inertial load is applied to the actuator within a backlash range of motion. Various profiles may be provided for transitioning between the first and second levels, and the gain levels may be adapted to the specific application and loading.
A system is also provided by virtue of the present technique. The system may include an actuator coupled to a machine and configured to drive the machine in response to control signals. A controller is coupled to actuator and is configured to apply control signals to the actuator, with the controller applying the control signals based upon a first gain level when a normal inertial load is coupled to the actuator, and reducing the gain level to a second level when a reduced inertial load is applied to the actuator within a backlash range of motion.
Again, a broad understanding of the term backlash is contemplated in accordance with the present technique. That is, the technique may be generally applied within a range of motion which is susceptible to sudden and significant variations in inertial loading, such as due to a change in contact two or more machine elements.